An optical method is utilized to determine the quantity of a specific component in a sample liquid such as urine or blood. In this method, a reaction system including a sample liquid and a coloring substance is irradiated with light, and amount of reflected light, transmitting light, or scattering light is measured as a response from the reaction system. The measured amount of light is compared with a predetermined calibration curve to calculate the concentration of the specific component.
In an optical method, a difference in the concentration of a specific component between different sample liquids needs to be reflected as a relatively large difference between measured amounts of light for providing a high resolution. Thus, light selected to be irradiated onto the reaction system needs to have a wavelength which shows large light absorbance at the reaction system (more specifically, a reaction product between the specific component and the coloring reagent). For example, p-nitroaniline or p-nitrophenol may be used as a coloring substance which is irradiated with light having a wavelength of 450 nm to determine the concentration of GGT (gamma glutamyl transpeptidase), ALP (alkaline phosphatase), and Amy (amylase) based on the response from the coloring substance. For this purpose, light emitted from a light source is caused to pass through a wavelength selecting filter where a light component having a specific wavelength is extracted for irradiation onto the reaction system.
An example of wavelength selecting filter is an interference filter. The interference filter utilizes interference occurring at a transparent thin film having a thickness nearly equal to a desired wavelength of light, for transmitting or reflecting a light component with the desired wavelength. The transparent thin film may be formed by vapor deposition for example. Therefore, even if an attempt is made to produce thin films, there will be some degree of variations in selectable wavelength due to thickness variations of the product films. Improvement in measurement accuracy requires an interference filter having a reduced variation in selectable wavelength. As a result, interference filters having a large extent of errors cannot be used, which causes a problem that the production yield of interference filters reduces to result in a cost increase.
On the other hand, an example of light source for light irradiation is an LED which emits light having a temperature-dependent wavelength. Thus, the wavelength of emitted light changes due to a rise of environmental measurement temperature or due to a temperature rise of the actuated LED itself. Such a change in wavelength ranges to about ±10 nm of the peak wavelength.
The above-described variation or change in the peak wavelength affects the measured amount of light from the reaction system. As schematically shown in FIG. 9 with respect to GGT for example, even if the absorbance is constant at the reaction system, the calculation result largely varies as the measurement wavelength varies. Therefore, the measurement accuracy lowers due to irregularities of the measurement wavelength. This fact can be seen also from the simulation graph shown in FIG. 10.
FIGS. 10A-10C illustrate the results of simulation with respect to GGT, ALP, and Amy, taking the relationship between wavelength deviations from the wavelength setting and measurement errors when the measurement wavelength is set at 405 nm. As seen from the figures, the measurement error increases as the deviation from the wavelength setting increases, regardless of the concentration of the measured component.